5G is expected to operate in a wide range of frequency bands, probably using also very high frequency bands compared to 4G. This implies, for example, lower diffraction and higher outdoor-to-indoor penetration losses, which means that signals will have more difficulties to propagate around corners and penetrate walls. Also, the initial deployment of 5G will be rather spotty.
The state-of-the-art integration between two different Radio Access Technologies, RATs, is hard handover, see e.g. Ref. 3GPP15-36300. The major drawbacks with inter-RAT hard handover, e.g. between 3G and 4G are the rather long delay and service interruption as well as the low reliability. A tighter integration with evolved Long Term Evolution, evolved LTE, may therefore be crucial in order to ensure ultra-high reliability and extreme bit rates in a 5G system.
One alternative is to use a solution similar to 5G dual connectivity, i.e. both User Plane, UP, and Control Plane, CP, are connected to both LTE and 5G and the UP data is aggregated, or split, at PDCP layer. Typically, this means the bearer split option referred to in the art as 3C, i.e. the bearer is split in the master eNB at Packet Data Convergence Protocol layer, PDCP layer. Dual connectivity increases the user throughput, due to UP aggregation, especially at low load and increases the reliability, due to CP diversity.
However, Dual Connectivity, DC, does not increase the coverage of the user plane data unlike solutions such as Coordinated Multi Point, CoMP, soft handover and multi-flow. All these solutions transmit the same UP data over all links and thereby increases the coverage. CoMP and Soft handover rely on a synchronized transmission, and reception, and maximum ratio combining, MRC, of the symbols. However, for LTE and 5G, sometimes referred to as New Radio, NR, this can be very difficult due to different transport formats, pilots, waveforms, numerology etc. Also, both CoMP and soft handover requires very good backhaul (X2) and quite synchronized networks. Therefore, solutions like HSPA multi-flow may be a solution for LTE-5G tight integration (and NR-NR multi-connectivity) enabling coverage extension due to selection ratio combing (SRC). This is not as good as CoMP/Soft handover which enables MRC but can still give large benefits especially in situations with very fast and varying channels.